1. Field of the Invention
The present invention relates to a process for preparing thermally insulating polymer foam, the thermally insulating polymer foam and a process for using a thermally insulating polymer foam.
2. Description of Related Art
There is an international desire to find and implement environmentally friendly blowing agents for preparing thermally insulating polymer foam. Environmentally friendly blowing agents have little or no detrimental impact on the environment. Two measures of detrimental environmental impact are Ozone Depletion Potential (ODP) and Global Warming Potential (GWP).
Hydrofluorocarbon (HFC) and fluorocarbon (FC) blowing agents have been one focus of recent thermally insulating polymer foam development due to their low thermal conductivity and their zero ODP relative to former blowing agents such as hydrochlorofluorocarbons (HCFC) and chlorofluorocarbons (CFC). However, even HFC and FC blowing agents tend to have GWPs higher than is desirable. For example, 1,1,1,2-tetrafluoroethane (HFC-134a) has a GWP of 1300 and 1,1-difluoroethane (HFC-152a) has a GWP of 140 (see United States patent application 2006/0202154, paragraph 25). An environmentally friendly blowing agent desirably has a GWP below 50. Therefore, exploration for a more environmentally friendly blowing agent continues.
Recent literature reveals that fluorinated olefins (fluoroalkenes) may be an attractive replacement for HFCs in many applications, including blowing agents, because they have a zero ODP, a lower GWP than HFCs, and high insulating capability (low thermal conductivity). See, for example United States patent application (USPA) 2004/0119047, 2004/0256594, 2007/0010592 and PCT publication WO 2005/108523. These references teach that fluoroalkenes can be suitable for blowing agents and are attractive because they have a GWP below 1000, preferably not greater than 75. USPA 2006/0142173 discloses fluoroalkenes that have a GWP of 150 or less and indicates a preference for a GWP of 50 or less.
Besides being environmentally friendly, a blowing agent for thermally insulating polymer foam desirably: (1) is sufficiently soluble in the polymer matrix of the foam so as to enable preparation of quality foam; (2) has a low thermal conductivity; and (3) has low permeability through (that is, has longevity in) the polymer matrix of the foam so to provide long-term thermally insulating capability.
Blowing agent solubility in the polymer matrix of a polymer foam is important in order to ensure several aspects of a quality foam. A quality polymer foam has an average cell size of 0.02 to 5 millimeters, is close-celled and has a density of 64 kg/m3 or less. If a blowing agent is insufficiently soluble in the polymer matrix it tends to result in a foam suffering from one or more of the following: a small average cell size, high density (greater than 64 kg/m3), high open cell content, and blowholes. (See, for example, teaching in PCT publication WO 98/03581 on page 12, lines 22-27). A quality foam also has a largely uniform cell size distribution. A quality foam is desirably essentially free of blowholes. If a blowing agent is too insoluble in the polymer matrix it may also cause blowholes as it rapidly expands out of the polymer matrix. Blowholes are voids the size of multiple cell diameters and are easily observed by the naked eye. Blowholes often cause an undesirably irregular foam surface as the blowing agent rapidly erupts through the foam surface during the foaming process.
Low thermal conductivity and high longevity in a polymer matrix is desirable to maximize a polymer foam's thermal insulating capability over time. A blowing agent having a high permeability in a polymer matrix will readily escape from a foam made from that polymer matrix. Therefore, it is desirable for a thermally insulating blowing agent to have a low permeability through the polymer matrix in which it resides.
USPA 2004/0119047, 2004/0256594, 2006/0142173, 2007/0010592 and PCT publication WO 2005/108523 each suggest that fluoroalkenes may be environmentally friendly and capable of forming a polymer foam. However, none of these references disclose if any of the fluoroalkenes actually have a GWP below 50, let alone whether any of the fluoroalkenes have a permeability in alkenyl aromatic polymers low enough to provide long-term thermal insulating capability or are capable of preparing a quality thermally insulating polymer foam.
Therefore, it remains desirable and would be surprising to identify a blowing agent with all of the following properties: a zero ODP, a GWP less than 50, a low thermal conductivity and low permeability through an alkenyl aromatic polymer matrix of a polymer foam, and a solubility in alkenyl aromatic polymers sufficiently high that the blowing agent can comprise more than 50 wt % of a blowing agent composition useful for producing a quality alkenyl aromatic foam.